Hi my name is Leo Rivera. I’m the Director of Scientific Outreach here at METER Group. And today we’re here to talk about particle size analysis measurements, and we’re going to focus on the basics behind the measurement.

So before we get into the measurements themselves, it’s really important to think about the importance of the particle size distribution of soils and why we even want to make this measurement. And one of those primary reasons is that the particle size distribution of soils is a fundamental soil physical property, and it impacts so many different things.

• It impacts the soil’s ability to retain water
• It impacts our hydraulic conductivity of soil, how water and nutrients leach through the

soil.

• It impacts the erosion potential of soil, plant nutrient storage dynamics, and especially things like organic matter dynamics and how that changes, because soil texture can play a really big role there.
• It impacts the carbon sequestration capacity of soils.

And typically in the US, we think of the soil as being kind of a function of the US Soil Taxonomy Textural Triangle. And typically we’re breaking it up into different classes of soil based on the sand, silt, and clay fraction.

And so as we’re thinking about that, it’s really important to think about what makes up particle size distribution. What is it that we’re actually looking at? When we break this down, we’re primarily looking at the mineral components of soil. We’re trying to figure out the breakdown in terms of a percentage of based on weight of the sand fraction, which is the 0.05 to 2.00 millimeter size soil minerals, the silt fraction which is the 0.02 to 0.05 millimeter size soil minerals, and then our clay fraction which is anything less than 0.02 millimeters.

That percentage breakdown is really important. That’s what really governs what our soil texture is and a lot of soil properties.

Traditionally, when we’re trying to make these measurements to figure out what our sand, silt and clay fractions are, these are some of the tools that we would normally use to make these measurements.

• Hand Texturing—The first, most basic tool for particle size measurements is hand texturing. You take a small, wet clod of soil and feather it out. You’re trying to estimate how much sand, silt and clay is in that soil by hand.
  As you can imagine, this is a very crude method. It’s not very accurate, but it’s a tool that we use in the field when trying to quickly assess what the soil is that we’re working with.
• Traditional Particle Size Analysis Methods— Other measurements of particle size analysis or soil texture are done using tools like sieving, where we sieve out the different sizes of soil particles using either dry or wet sieving. Typically, that’s combined with a sedimentation method, such as the hydrometer or pipette method. These are based on Stokes’ law.
• Modern Particle Size Analysis Methods— Some of the newer methods that we’ve also have seen come into use include the optical methods like X ray attenuation, laser diffraction, and visible near infrared spectroscopy. We’re primarily just going to focus on the laser diffraction today, but there are other methods for making these measurements optically as well.

An important thing to think about when it when we’re making these measurements the classification system that we’re trying to base our measurements on. We have the USDA soil classification system, but there are also ASTM standard methods that used on the engineering side. You have the unified soil classification system, and Germany has its own classification system.

Each classification system breaks down different sizes for different minerals that we have to think about when making these measurements. So again, it’s important to think about the application that you’re actually trying to use these measurements for, and use that as your guiding light for what measurement is going to work best for you, because not every measurement method is going to work best for each application, and sometimes one might be overkill compared to another. So think about this when it comes to what measurement you’re thinking about using, and which measurement method will work best for you.

Some other fundamental pieces behind the measurement that are really important to think about are how you are trying to portray the data that you’re measuring. Oftentimes, when we’re thinking about particle size analysis measurements, the measurements are typically given in there in terms of soil texture or your sand, just your basic breakdown of your sand, silt, and clay fraction, but also sometimes we’re trying to actually look at the particle size distribution curve like you see in this chart. And there are advantages to both methods.

Of course, basic soil texture is super simple and and really easy to portray, but there are some components that you can get out of the particle size distribution curve that are helpful when understanding what the actual breakdown of the sand, silt, and clay fraction, and as those those sizes change, what that actually might mean in terms of some of these soil mechanics and the behavior that you’re going to see in terms of hydraulic properties and all of these things that you might see in soil.

One last piece to think about is the soil handling that you have to go through when making these measurements. Almost all of these measurement methods, minus some of the optical methods, involve some preparation that has to be done to the soil before making the measurements, and this involves dispersing the soil into individual primary particles. So typically, we use a chemical and physical dispersant. Oftentimes, we have to remove organic matter from the soil to make this measurement, and in some cases, we’re gonna have to use an iron oxide and carbonate remover. Those situations are rare, but these are things that you will have to consider, because these might impact your ability to make these measurements.

So before we get a little bit deeper, it’s really important to understand some of the physics behind this measurement and what we’re doing. The most common measurement methods for doing particle size analysis are based on Stokes’ law. Sedimentation methods involve taking an aqueous suspension in a cylinder, and you can kind of see some examples of that in this image. After homogenizing the particles—essentially, that means getting the particles on suspension equally—we measure the settling velocity of that solution. That depends strongly on the size of the particles that are in suspension. The larger particles fall out faster, while sands and finer particles like the clays are going to stay in suspension longer. In some cases, they stay in suspension indefinitely. And Stokes’ law relates that settling time of the size of the particles that remain in suspension. That’s how most of these measurement methods are working to determine what is actually remaining in suspension at different time intervals. We’ll talk more about the those methods and how they work in a future video.